Chemical treatment of oil well liquids for preventing scale formation



' non-scale-forming.

of alkaline ea I Patented 1947 UNITED STATES PATENT OFFICE CHEMICAL TREATMENT OF OIL WELL LIQUIDS FOR PREVENTING SCALE FORMATION Leslie Cline case. Tulsa, Okla., asslgnor to Gulf Oil Corporation, Pittsburgh; Pa, a corporation No Drawing. Application January 2, 1946,, Serial No. 638.709

4 Claims. (CL 2528.55)

This invention relates to the chemical treatment of oil well liquids for the prevention of scale formation on bore hole equipment and more particularly to the use of a-substantially saturated aqueous solution of sodium chloride con- I taining an alkali metal polyphosphate dissolved therein for treating oil well liquids to make them In order that this inventionmay be more fully understood I shall first give a brief description of the bore hole equipment used in an oil well and then point out how the problem of scale formation arises in the operation of such a well.

Producing oil well subsurface equipment comprises an outer casing and an inner tubing through which the oil well liquids are produced. The casing is cemented or sealed for some distance above the oil zone so that water from ad- 1 jacent rock formations may not contaminate the well fluids. Several casings are sometimes necessary to seal oil water and to facilitate drilling of the well. In such a case, only the innermost or smallest casing extends to a depth near the.,oil zone where (it is cemented. The tubing, or production pipe, is not cemented but is chiefly supported at the well head and thus has no obstruction at the bottom. From this it is readily seen that when liquid is introduced at the well head into the annulus between the tubing and the easing, it will travel to the bottom of the well to be produced with the well liquids through the tubing. Since this annulus is open to the oil-producing formation, a force pump must be used to inject a treating solution when the well is flowing by its own pressure. If, on the other hand, the well is being produced by a pump at the bottom of the tubing, the liquid level in the well may be 'near bottom and only moderate or no pressure will be present in the annulus. Under the latter conditions a, treating solution may be fed by gravity into the annular space between the tub= ing andthecasingr 1 Scale deposition takes place mainly on bottom lligilie pump parts and on the inside of the tubing.

s scale appears to have two modes of origin. supersaturation occurs in the oil well brine I leaves the oil formation and enters the p ping mechanism under reduced pressure.

Two" incompatible compounds are com d. by means of a casing leak which allows t to come in contact with the oil well Ingthe case the scale is composed mainly admixtures of iron carbonate, iron sulfide, iron rth carbonates and sulfates, with z oxide, silica and fine sand, silt or other sediment. Physical causes of the reactions are'thought to be loss of heat, loss of pressure. evaporation and agitation. Loss of pressure releases carbon dioxide from calcium. bicarbonate and ferrous bicarbonate causing scale containing calcium carbonate and ferrous carbonate to deposit. Calcium sulfate in solution is transformed into calcium sulfate scale by evaporation of water and loss of heat. W

In the second case the scale is composed mainly of alkaline earth sulfates sometimes with carbonates and a smaller portion of inert material. If two waters, carrying incompatible compounds, ,are mingled in a well bore. precipitation of solid material will result as indicated by the following equations:

Calcium carbonate scale will also deposit when fresh water from near the surface saturated with calcium bicarbonate leaks into the well bore and mingles with a salt brine.

It is accordingly an object of this invention to provide a solution for "down the hole" treatment of oil well liquids which will counteract the formation of scale. A further object is to provide a solution that will inhibit the deposition of scale On bore hole equipment from oil well liquids. A still further object is to provide a method of treating oil well liquids to retard scale formation on metal equipment in the bore hole. Other objects will appear hereinafter.

These objects are accomplished in accordance with the present invention by utilizing a substantially saturated aqueous solution of sodium chloride containing an alkali metal polyphosphate dissolved therein for "down the hole" treatments of oil wells to counteract scale formation on metal equipment. In this specification and in the sub- -joined claims when I refer to a substantially phosphate (NaaP'zoz-z), which are all genetically derived by molecular dehydration of orthophosphoric acid compounds.

Strong solutions of sodium hexametaphosphate are very corrosive. However, treating solutions that are made by dissolving sodium hexametaphosphate in sodium chloride brine have greatly reduced corrosive characteristics so that they may be used without causing any corrosion troubles. The ability of sodium chloride to inhibit the corrosive activity of solutions of sodium hexametaphosphate is one of the advantageous features of the present invention.

In preparing treating solutions for use in the present invention I find it convenient to mix a known volume of a saturated aqueous sodium chloride solution with a known volume of an aqueous solution of an alkali metal polyphosphate having a predetermined concentration. In actual practice I have prepared a saturated sodium chloride solution which has a specific gravity of 1.203 and which contains 26.3 per cent by weight of sodium chloride by allowing water to stand in contact with sodium chloride for two days and shaking occasionally. Various ,treating solutions useful in the practice of the present invention are made by mixing a known amount of this saturated sodium chloride solution with a given amount of a solution containing 5 pounds of sodium hexametaphosphate per gallon. The following table gives the properties of treating solutions prepared by mixing described in the above table freeze, salts are not thrown down which fail to redissolve when the temperature increases slightly.

In preparing and using a treating solution containing an alkali metal polyphosphate to prevent-the accumulation ofscale.,deposits,,it is desirable to observe the followingeprecautions:

(a) The solution should be of sufiicient volume to be adjusted to proportionate mingling with the oil well fiuid, but the volume should not exceed that easily transported 0r handled by small chemical pumps at the well.

(b) The solution should not freeze at prevailing temperatures in the winter time.

The treating requirements of oil well brines are variable and are determined to some extent by laboratory tests and to some extent by tests at the well. In general,0.2 pound of alkali metal polyphosphate per 100 barrels of eiiluent brine is required to suppress soft scale formation, consisting mainly of alkaline earth carbonates. Approximately 0.4 pound of alkali metalpolyphosphateper 100 barrels of effluent brine is required to. suppress hard scale formation, consisting essentially of alkaline earth sulfates. Thus, the polyphosphate requirement will be within the approximate range of 5.7 to 11.4 milligrams per liter of the oil well brine produced. It will be noted a saturated sodium chloride solution with a sodium hexametaphosphate solution. Sodium chloride-sodium hemametaphosphatewater mixtures [Solution No. 1-Saturated NaOl, sp. gr. 1.203] [Solution No. 2(N8POs)s-5 lbs./ge1., sp. gr. 1.376]

Per Cent Per Cent Volumes of Volumes of (b Freezing y wt.) (by wt.) I 9 Solution 1 Solution 2 N801 (NBPOI) Ioint, F.

As can be seen from the above table a very economical and practical solution for anti-scale treatment may be made by mixing 8 volumes of a saturated sodium chloride solution made by allowing fresh water to stand in contact with commercial sodium chloride for several days with occasional stirring, with 1 volume of a solution made by adding sodium hexametaphosphate to fresh water in the amount of 5 pounds per gallon of water. This treating solution contains 0555 pound per gallon of sodium hexametaphosphate and freezes only slightly at 13 F.

Sodium septaphosphate 'or other alkali metal polyphosphates mentioned above may be used in place of sodium hexametaphosphate in the preparation of treating solutions similar to the ones described above. Small amounts of alkali may be used with these treating solutions if some anticorrosion treatment is desired along with the anti-scale treatment. In preparing these treating solutions it is desirable to separately dissolve the sodium chloride and the alkali metal polyphosphate since the polyphosphate. does not readily dissolve in a saturated aqueous sodium chloride solution. When the treating solutions that the regulation of oil well treatment is facilitated by proper choice of one of the treating solutions described in the above table. For example, if it is desired to inject 0.1 pound polyphosphate daily into a well which produces barrels of brine per day, the 11:1 dilution may be conveniently used as a treating solution. One gallon of this treating solution contains 0.416 pound polyphosphate, and hence one quart thereof will furnish the daily requirement of 0.104

pound polyphosphate. As previously mentioned, the treating solution may be varied within this desirable range to furnish proper amounts of polyphosphate to suppress the formation of the particular type of scale in a given amount of oil well brine produced.

The proper rate of treating solution injection may be attained either with a chemical pump or by allowing the treating solution to drip by gravity from a small drum into the wellx-head. A chemical pump is used when an oil W611"iS=.fiOW- ing under natural pressure and thezgravity system is used when the oil well is being pumped or not produced under pressure.

The question as to whether the treating solu* tion shall be added continuously or periodically to the. well liquids can only be determined by conducting separate tests to determine the most economical method of carrying out the treatment at each well. Despite the fact that a periodical treatment of the well liquids with the treating agent resultsin extensive periods in which there is a low concentration of alkali metal polyphosphate inthe well'liquids, in general it has been found that in the long run the periodic rather than the continuous method of carrying out the treatment is most economical.

The treating solution may be added to the well in various" ways. The determination of the manner of adding'the treating solution depends largely on the number of hours that the well. is pumped each day. Thus the procedure generally adopted in treating wells which are pumped from two to twelve hours per day consists in adding, a

predetermined amount of treating solution to. the well just after the 'shut down of the pumpgand Will ng down the solution with about.one bar-.

water. For treating wells having a pumping period of ten to twelve hours a day a combination method of treatment is generally used, comprising the continuous addition of a regulated small amount or treating solution during the course of the pumping period followed upon shut down by an additional charge which is washed down with salt or fresh water in amount suflicient to provide treatment for the incoming water at the bottom of the well during the period in which the pump is idle.

It has been found that pump cups, whether of leather or composition, have a much longer life in wells treated by the method of the present invention, since only soft, fine grained precipitates are formed which are pumped out of the well readily without injuring or scoring the cups or the barrel. The cups in wells which are not treated often last but a short time, due to abrasive action of the hard scale flakes or chips formed on the metal parts adjacent the cups and jarred loose during the operation of the pump. The barrel of the pump and the cups are not lubricated by the present chemical treatment but the cups and working barrel are subjected to service under conditions substantially normal to a well not troubled with hard scale formation. The life of the cups, balls, valve seats and barrels in wells treated in accordance with the method outlined above is noticeably increased thereby, and the savings due to such increased life are large.

Resort may be had to such modifications and variations as fall within the spirit of the invention and the scope of the appended claims.

What I claim is:

1. A method of preventing oil well liquids from depositing scale on borehole equipment which comprises treating said oil well liquids in the well with a substantially saturated aqueous solution of sodium chloride containing an alkali metal 6 polyphosphate dissolved therein, the amount of alkali metal polyphosphate being sumcient to inhibit the deposition of scale on said equipment.

2. A method of preventing oil well liquids from depositing scale on bore hole equipment which comprises treating said oil well liquids in the well with a substantially saturated aqueous solution of sodium chloride containing sodium hexametaphosphate dissolved therein, the amount of sodium hexametaphosphate being sufficient to inhibit the deposition of scale on said equipment.

3. A method of preventing oil well liquids from depositing scale on bore hole equipment which comprises treating said oil well liquids in the well with a substantially saturated aqueous solution of sodium chloride containing sodium septaphosphate dissolved therein, the amount of sodium septaphosphate being sufiicient to inhibit the deposition of scale on said equipment.

4. A method of treating oil wells to retard scale formation on metal equipment in the bore hole which comprises introducing a substantially saturated aqueous solution of sodium chloride containing dissolved therein from about 3 per cent to about 8 per cent by weight of sodium hexametaphosphate into the well near the bottom of the production pipe.

LESLIE CLINE CASE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,829,705 Walker Oct. 27, 1931 1,873,084 Walker Aug. 23, 1932 1,997,256 Hall Apr. 9, 1935 2,035,652 Hall Mar. 31, 1936 2,156,173 Bird Apr. 25, 1939 OTHER REFERENCES 

